A) Global warming isn't going to kill the Earth. It, and the life it supports, has survived a few years in the past without mankind, and it will do it again.
There is a possibility that Earth is the only planet with advanced life, yes, but it is a pretty slim one. Heck, who says advanced life has to live on a planet? Or within the confines of our universe, or in a universe at all? Throughout our history, humans have overestimated the relative prominence of the world that was known to them vs. the world that was not. It may very well be a mistake to think that this tiny ball of dust is more important and unique on a cosmic level than it really is.